An impeller shell and a converter cover of a conventional torque converter are typically assembled using an arc welding operation. However, when using such an operation, one problem that usually occurs is sputters that are generated during the operation. More specifically, the sputters may sneak through a gap of a fitting portion disposed between the impeller shell and the converter cover. Thus, the sputters may inadvertently penetrate into the torque converter and turn into metallic granular substances. As such, the metallic granular substances may become prevalent in the ATF (Automatic Transmission Fluid), thereby requiring a high-performance filter with a fine mesh to filter out same.
In one attempt to resolve the above problem, Japanese Laid-Open Patent Publication No. 2004-286105 discloses a method of assembling a torque converter using a friction stir welding operation. Specifically, the fitting portion of an impeller shell and a converter cover is faced toward a stir rod (tool) to implement the above operation, wherein the stir rod transitions along an axial direction of the torque converter. The stir rod is then inserted into the fitting portion to thereby bond them together without generating any sputters.
In the above method, the bonding portions of the impeller shell and the converter cover are formed in a planar shape. By doing so, the stress at the bonding portion caused by an inner pressure of the torque converter can be avoided. In this respect, a spacing portion for forming a planar surface is formed at a lower surface side of the bonding portion (see cavity 13 shown in FIG. 3 of Japanese Laid-Open Patent Publication No. 2004-286105).
However, when using the above method, a plastically flowing material may flow into the spacing portion during the friction stir welding operation. Further, a depression may occur at the bonding portion, thus deteriorating the overall bonding strength of the bonding portion.